Concrete mixer



FIG. 1 is a left side view of a concrete mixer, showing my new design, the right side being identical;

FIG. 2 is a top view;

FIG. 3 is a rear end view;

FIG. 4 is a front end view;

FIG. 5 is a left side view thereof, shown in a tilt position. 

The ornamental design for a concrete mixer, as shown and described. 